I propose the following errata:Cards placed off to the side are in play for the purposes of uniqueness. Unless stated
otherwise, when a host permanent-event is removed from the playing surface, any
cards placed off to the side under it are discarded.
"Cards placed off to the side are always in play for the purposes of uniqueness. Unless stated
otherwise, when a host permanent-event is removed from the playing surface, any
cards placed off to the side under it are discarded."
Change in bold.
Rationale:
" if it is not there it is not there"
Dark Minions: Great Secrets Buried There
Rarity: Uncommon, Precise: U2
Hazard: Permanent-event
Playable if opponent has at least ten cards in his play deck. Opponent reveals the top ten card of his play deck to himself. If one is available, opponent must choose a non-special, non-hoard item from the revealed cards to place off to the side under this card (item does not give Marshaling points and is considered out of play). If none are available, opponent must show you the cards he revealed to himself. Opponent shuffles all remaining revealed cards into his play deck. Opponent may play this item as though it were in his hand at any Under-deeps site. Alternatively, you may play this card as a resource on yourself if you have at least ten cards in your play deck. In this case, you and your opponent reverse roles.
Great Secrets Buried There does not specify any exception from "and is considered out of play". Therefore it supersedes "Cards placed off to the side are in play for the purposes of uniqueness." completely.
I think that the phrase "and is considered out of play" has a purpose. The purpose is not an allowing for second copy/manifestation of an item in play. The purpose is a removing the effects on play that the item would have otherwise.
For example, if Palantír of Amon Sûl would be placed off to the side, without this phrase, its effect could be duplicated by Palantír of Osgiliath.